Noisy Workhorse engine fan

Recognize that the fan clutch on/off is based on the air temperature flowing across the fan blades as the engagement is determined by a wax motor - ie, no "direct" connection to engine coolant temperature (unless you have electric clutch then may be ECM controlled by coolant temp). Making sure you have good air flow over and through radiator is best course and making sure shroud directs all the air through and over fan clutch will make sure it is getting proper air flow.
I would also recommend putting OEM temp stat back in, will slightly improve fuel economy.